Fuel cells are chemical power sources in which electrical power is generated in a chemical reaction. The most common fuel cell is based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. The consumption of these agents is proportional to the power load. Because hydrogen is difficult to store and distribute and because hydrogen has a low volumetric energy density compared to fuels such as gasoline, hydrogen for use in fuel cells will have to be produced at a point near the fuel cell, rather than being produced in a centralized refining facility and distributed like gasoline.
Hydrogen is widely produced for chemical and industrial purposes by converting materials such as hydrocarbons and methanol in a reforming process to produce a synthesis gas.
Synthesis gas is the name generally given to a gaseous mixture principally comprising carbon monoxide and hydrogen, but also possibly containing carbon dioxide and minor amounts of methane and nitrogen. It is used, or is potentially useful, as feedstock in a variety of large-scale chemical processes, for example: the production of methanol, the production of gasoline boiling range hydrocarbons by the Fischer-Tropsch process and the production of ammonia.
Processes for the production of synthesis gas are well known and generally comprise steam reforming, auto-thermal reforming, non-catalytic partial oxidation of light hydrocarbons or non-catalytic partial oxidation of any hydrocarbons. Of these methods, steam reforming is generally used to produce synthesis gas for conversion into ammonia or methanol. In such a process, molecules of hydrocarbons are broken down to produce a hydrogen-rich gas stream.
Modifications of the simple steam reforming processes have been proposed. In particular, there have been suggestions for improving the energy efficiency of such processes in which the heat available from a secondary reforming step is utilized for other purposes within the synthesis gas production process. For example, processes are described in U.S. Pat. No. 4,479,925 in which heat from a secondary reformer is used to provide heat to a primary reformer.
The reforming reaction is expressed by the following formula: EQU CH.sub.4 +2H.sub.2 O.fwdarw.4H.sub.2 +CO.sub.2
where the reaction in the reformer and the reaction in the shift converter are respectively expressed by the following formulae (1) and (2) EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 EQU CO+H.sub.2 O.fwdarw.H2+CO.sub.2
In the conventional hydrogen generating apparatus, an inert gas heated in a reformer is made to flow through a process flow path so as to raise temperatures of the shift converter and the heat exchangers which are downstream from the reformer.
U.S. Pat. No. 5,110,559 discloses an apparatus for hydrogen generation which includes a reformer and a shift converter each incorporating a catalyst wherein, during the start-up of the apparatus, reformer combustion gas is introduced to a shift converter jacket surrounding the shift converter catalyst to heat the shift converter to provide a start-up or temperature rise of the reformer system.
U.S. Pat. No. 4,925,456 discloses a process and an apparatus for the production of synthesis gas which employs a plurality of double pipe heat exchangers for primary reforming in a combined primary and secondary reforming process. The primary reforming zone comprises at least one double-pipe heat exchanger-reactor and the primary reforming catalyst is positioned either in the central core or in the annulus thereof. The invention is further characterized in that the secondary reformer effluent is passed through which ever of the central core or the annulus is not containing the primary reforming catalyst countercurrently to the hydrocarbon-containing gas stream.
U.S. Pat. No. 5,181,937 discloses a system for steam reforming of hydrocarbons into a hydrogen rich gas which comprises a convective reformer device. The convective reformer device comprises an outer shell enclosure for conveying a heating fluid uniformly to and from a core assembly within the outer shell. The core assembly consists of a multiplicity of tubular conducts containing a solid catalyst for contacting a feed mixture open to the path of the feed mixture flow such that the path of the feed mixture flow is separated from the heating fluid flow in the outer shell. In the process, an auto-thermal reformer fully reforms the partially reformed (primary reformer) effluent from the core assembly and supplies heat to the core assembly by passing the fully reformed effluent through the outer shell of the convective reforming device.
Fuel cells are chemical power sources in which electrical power is generated in a chemical reaction. The most common fuel cell is based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. The consumption of these agents is proportional to the power load. Because hydrogen is difficult to store and distribute and because hydrogen has a low volumetric energy density compared to fuels such as gasoline, hydrogen for use in fuel cells will have to be produced at a point near the fuel cell, rather than be produced in a centralized refining facility and distributed like gasoline. Polymers with high protonic conductivities are useful as proton exchange membranes (PEM's) in fuel cells. Among the earliest PEM's were sulfonated, crosslinked polystryenes. More recently sulfonated fluorocarbon polymers have been considered. Such PEM's are described in an article entitled, "New Hydrocarbon Proton Exchange Membranes Based on Sulfonated Styrene-Ethylene/Butylene-Styrene Triblock Copolymers", by G. E. Wnek, J. N. Rider, J. M. Serpico, A. Einset, S. G. Ehrenberg, and L. Raboin presented in the Electrochemical Society Proceedings (1995), Volume 95-23, pages 247 to 251.
The above processes generally relate to very large industrial facilities and the techniques for integrating the steps of converting the hydrocarbon or alcohol feedstream may not be useful in compact, small-scale hydrogen-producing units to power a transportation vehicle or to supply power to a single residence. One of the problems of large hydrogen facilities is the problem of methane slippage in steam reforming reactors. Methane slippage is a term used to describe a reduction in the methane conversion across the reforming reactor.
Generally, the equilibrium conversion of methane to carbon oxides and hydrogen that is achieved in the reforming reactor increases with temperature. Consequently, a reduction in the reactor outlet temperature corresponds to a lower conversion of methane, or a methane slippage. Methane slippage reduces the overall production of hydrogen and hence the efficiency of the process. Methane slippage can create problems in downstream equipment such as in an oxidation step used to remove trace amounts of carbon monoxide from the hydrogen stream before passing the hydrogen stream to the fuel cell.
It is the objective of this invention to provide a compact apparatus for generating hydrogen from available fuels such as natural gas, hydrocarbons, and alcohols for use in a fuel cell to generate electric power.
It is an objective of this invention to provide an integrated fuel cell and hydrogen production system which is energy and hydrogen efficient.
It is an objective of the present invention to provide an apparatus for the steam reforming of methane which mitigates the methane slippage problem and achieves a more uniform temperature throughout the steam reforming zone.